Any exercise that raises your heart rate will improve your health, but high intensity interval training (HIIT) is especially good because it rebuilds cell organelles responsible for energy production, U.S. researchers report. HIIT pushes cells to make more proteins for the energy-producing mitochondria – actually reversing cell aging, something that cannot be done using drugs or medicines. Volunteers in the study included 36 men and 36 women: half 18-30 years old and half 65-80. One group did HIIT biking, one did strength training with weights, and one did both. Strength training improved muscle mass in young and older people. But younger volunteers in the HIIT group saw a 49 percent increase in mitochondrial capacity; older HIIT volunteers saw an impressive 69 percent increase.